Here is my guess:
There is no need to reserve a number of ITL because it is always possible to split a block in order to have free space. So initrans is ignored.
However, the transaction that splits the block must have an ITL entry for itself, so I guess that oracle always keep one free ITL entry in order to be sure that a transaction can split the block.

So here is what I guess that happens:
1- the insert needs 1 ITL entry, and adds 1 free ITL free, so result is 2 ITL entries
2- when the block was split, ITL entries were copied in the two resulting blocks, so both blocks have 2 ITL entries
3- each concurrent insert need their own ITL (so 10) and keep 1 free entry, so result is 11.
After the block split, they are copied in both blocks.